Accelerator for chemical ripening



Feb. 25, 1969 ACCELERATOR FOR CHEMICAL G. H. KLINGER ET AL RIPENING Filed May 25, 1966 United States Patent O 6 Claims The invention is concerned with the ripening of photographic emulsions and in particular is based on the discovery that certain polyhydroxy compounds, added at a certain point in emulsion preparation can increase the rate of ripening of such emulsions.

Emulsions sensitive to light and other actinie radiation are conventionally prepared for photographic application by first soaking a part of the gelatin required for the emulsion formula in Water, completing its solution and then adding soluble halide salt, usually with addition of ammonia. Such halide salts may be straight chlorides, bromides or iodides, usually in the alkali metal form or may be chlorobromide, bromoiodide, etc., mixtures. Generally, mixtures of about 2-10 mole percent of alkali iodide and 90-98 mole percent of alkali metal or ammonium bromide or from 10-30 mole percent of alkali metal or ammonium chloride and 70-90 mole percent of alkali metal bromide are preferred.

To the resulting solution, silver nitrate is added in the desired concentration with the rate of addition, temperature and so on, being dependent on the particular emulsion product to be obtained. Generally, the silver is precipitated as the halide in this step. The partial emulsion so prepared is then heated to carry out a first ripening step. Additional gelatin is later added and finally, after the mixture has been set to a gel by chilling, it is broken up and washed to remove the excess salts. Breaking up to a condition finely divided enough for effective washing may be accomplished by a number of techniques, for example, by extrusion through a die to give slender threads (noodles) of gelatin, by a coagulation Wash technique employing generally organic wetting agent-type materials in which the gelatin is loosely coagulated and separated for washing, or by a precipitation technique `generally using inorganic salts, in which the gelatin is precipitated as relatively large particles suitable for washing. In both these latter techniques relatively dilute gelatin solutions are employed.

After washing, the emulsion generally is reconstituted by agitation and the addition of further gelatin and Water. The reconstituted emulsion is allowed to digest for a period of time and then other ripening agents for pH adjustment, sensitization, stabilization, etc. are added and mixture allowed to stand for a period of time. Following this after-ripening, coating finals, such as surfactants and wetting agents may be added before application of the emulsion to photographic sheet material.

Polyhydroxy compounds of higher molecular Weights, such as the dextranes and other sugar derivatives are known to affect the covering power and in some systems also the rate of development When added as coating finals. The molecular weight of these compounds ranges from about 300 to as much as 100,000. The effects are only noticeable if a certain minimum amount of more of the material is present. The lower limit is normally based on the gelatin present in the emulsion. A distinct disadvantage of the aforementioned compounds is their detrimental effect on the coating and hardening properties of the emulsions when added in such large amounts.

It has now 4been found that such polyhydroxy compounds may be used in much lower amounts and still achieve their effects when added before or during the pre-wash coagulation of the emulsion. Also, the addition lCe of these materials before or during the coagulation has been found to speed the after-ripening step, apparently by increasing the rate of ripening.

The polyhydroxy compounds useful in this invention generally have a molecular weight of about 180-300,000 and are organic materials having a sufficient number of hydroxyl or other polar groups to assure water solubility. As mentioned, low-value sugars and sugar derivatives such as pentaerythrytol, sorbitol, d-mannitol, dextran and glucoheptanoic acid are preferred. The amount of active agent employed is generally about 2-4% of the total gelatin present in the finished emulsion. If the same amount of a given polyhydroxy compound is added after the washing is completed, little or no effect is observed on the rate of ripening. It has been found that even compounds of low molecular weight with an excellent water solubility are not removed during the washing of the emulsion subsequent to coagulation.

The following examples of the process of this invention are to be considered illustrative only and not limiting.

Batches of gelatin emulsion were made containing 22 gra-ms KI, 680 grams NH4Br and 40 grams gelatin to 3600 mls. of water. Silver solutions containing 1000 grams AgNO3, 180 mls. of 10% NHO3 and 900 mls. of 28% NH3 in 5200 mls. of water were also prepared and added, during the course of l5 minutes, to the gelatin solution. Each mixture was allowed to digest for 5 minutes at 50. Then 8 grams of sodium polystyrene sulfonate coagulant as a l5 perecent aqueous solution and 2.5 grams of polyhydroxy compound, as reported in Table I below, were added. The silver halide crystals were precipitated and the coagulated mixture washed to remove excess salts. To each batch of coagulated gelatin, 50 grams of ripening gelatin is added and each finished emulsion is given a conventional after-ripening at 50 C. before being coated on a photographic medium, exposed and developed.

Representative results of sensitometric tests and measurements of fog and speed are given in Table I.

TABLE I After- Compound Ripening Fog Dm Speed Time at 0.2 (minutes) Blank 30 06 15 60 08 72 11 09 l. 83 52 d-Mannitol 30 08 27 60 09 1. 44 39 90 11 3. 16 85 Sorbitol 30 07 44 11. 5

60 08 1. 42 46 90 10 3. 44 110 Dextran 30 07 22 60 l0 l. 92 68 90 18 4+ 170 Pentaerythrytol 30 O7 60 10 60 14 l. 52 120 90 14 2. l5 220 Glucuheptanoic acid 30 .08 .92 45 60 14 1. 77 195 90 29 2. 04 260 The results of these runs are shown graphically in the drawings, where, in FIGURE l, the speed of the resulting photographic medium is shown in relation to after-ripening time of the emulsion while in FIGURE 2 fog is shown in relation to after-ripening time. The results obtained with -mannitol were essentially identical with those shown for sorbitol.

It can be readily seen that the use of all of the agents of the runs resulted in increased speed of the photographic medium while fog was not significantly increased, especially when using the 60 minute after-ripened products. The sorbitol and the mannitol were of particular value in the 90 minutes after-ripened product, without too significant an increase in fog.

3 4 What is claimed is: 5. The process of claim 1 in which the compound is 1. In a process for preparing a light-sensitive gelatinpentaerythrytol. silver halide emulsion including precipitation, digestion, A6. The process of claim 1 in which the compound is coagulation, washing and ripening periods, the step which glucoheptanoic acid.

comprises adding to the emulsion, lbefore the end of the coagulation step, about 2 to 4%, based on the weight of 5 References Cited the gelatin, of a water-soluble polyhydroxy compound UNITED STATES PATENTS having a molecular weight of about 180 to 300,000 and 3,063,838 11/1962 Jennings 96 107 XR selected from the group consisting of a d-mannitol, sorbi- 3,085,009 4 /1963 Chambers 96 107 tol, dextran, pentaerythrytol and glucoheptanoic acid. 10 3,272,631 9/ 1966 Garrett et al. 96-107 2. The process of claim 1 in which the compound is d-mannitol. I. TRAVIS BROWN, Prz'm'ary Examiner.

3. The process of claim 1 in which the compound is RONALD H' SMITH, Assistant Examnm sor-bitol.

4. The process of claim 1 in which the compound is 15 U.S. C1X.R. dextran. 96-107, 114 

1. IN A PROCESS FOR PREPARING A LIGHT-SENSITIVE GELATINSILVER HALIDE EMULSION INCLUDING PRECIPITATION, DIGESTION, COAGULATION, WASHING AND RIPENING PERIODS, THE STEP WHICH COMPRISES ADDING TO THE EMULSION, BEFORE THE END OF THE COAGULATION STEP, ABOUT 2 TO 4%, BASED ON THE WEIGHT OF THE GELATIN, OF A WATER-SOLUBLE POLYHYDROXY COMPOUND HAVING A MOLECULAR WEIGHT OF ABOUT 180 TO 300,000 AND SELECTED FROM THE GROUP CONSISTING OF A D-MANNITOL, SORBITOL, DEXTRAN, PENTAERYTHRYTOL AND GLUCOHEPTANOIC ACID. 